This invention relates to a mold forming material used in casting operation and more particularly it relates to an investment material for forming molds to produce precision castings, such as dental castings.
For example, in dental casting, it has been common practice to use pure titanium or titanium alloys as casting materials. And as mold forming materials in this case it has been proposed to use, firstly, phosphate-bound investment materials whose components are quartz (SiO.sub.2), cristobalite (SiO.sub.2), phosphates and magnesia (MgO). Secondly, it has been proposed to use materials whose main components are thermodynamically relatively stable oxides such as alumina (Al.sub.2 O.sub.3), zircon (ZrSiO.sub.4), zirconia (ZrO.sub.2), calcium oxide (CaO), and magnesia.
However, in the case of quartz and cristobalite in said first mold forming materials, when titanium is used as a casting material, the mold tends to be wetted by and react with the molten titanium. Thus, where said first mold forming materials are used to perform casting using titanium as a casting material, there has been a problem that the resulting castings tend to have casting surface defects and gas-caused defects.
Further, if said second mold forming materials are used instead of said first mold forming materials, said drawbacks become less frequent, but since said second mold forming materials cannot compensate for shrinkage which takes place during solidification of titanium, there is a problem that the resulting casting is smaller in size than required.
Thus, recently, a superior method has been proposed. According to this method, zirconium powder is added to mold forming materials to utilize the fact that zirconium oxidizes and expands during heating and firing; thus, shrinkage during solidification is compensated for. However, since zirconium is very difficult to refine, it is expensive; thus, there are problems concerning its practical use.
A study is also being made of the addition of other metal powders, but there are such problems as formation of bubbles due to reaction between metal and water, cracks occurring during heating and firing, and casting surface defects. Thus, such metal powders have not been put into practical use.